Wednesday April 7th 1999
Announcements:

Lecture notes:

Class 35

As usual the test will focus on the main most important ideas rather than details.

Will cover both telescopes and cosmology.

I have put up a list of key vocabulary words.  You should understand what these words mean and how they fit together.  This
will be a good start for studying for the test.  All these words are covered in the textbook but I would be happy to go over any
of them if there are questions.

Constellation Review
Virgo - Spica
Corvus
Leo
Perseus
Cassopeia
Cepheus
Andromeda
Canis Major - Sirius
Canis Minor - Procyon
Orion - Betelgeuse Rigel
Auriga
Gemini

Key Vocabulary Words
refracting telescope
reflecting telescope
Hobby - Eberly telescope
charge coupled devices
atmospheric "seeing"
interferometer
resolving power
segmented mirror
spectrograph
cosmology
homogeneity assumption of cosmology
expansion of the Universe
the Big Bang model
Hubble's Law
Olbers' paradox
cosmic microwave background
blackbody
COBE satellite
Intrinsic microwave background anisotropy
recombination
primordial nucleosynthesis
critical density
open universe
flat universe
closed universe

Telescopes
 devices used to study electromagnetic radiation - not only optical light here (also radio IR UV X-ray and gamma ray).  We covered optical telescopes first.  Have three purposes.

• - increase light gathering power
• - resolve fine details of cosmic objects
• - magnify images so we can see them.

Two types.
Refracting telescopes - use a large lens to gather and focus the light.
Reflecting telescopes - use a concave mirror to gather and focus the light

focal length - distance from lens/mirror to image formed of a distant light source.  Primary lens/mirror and eyepiece.

When people speak of telescopes they often make reference to the diameter of the mirror/lens.  For example a 2 meter reflector has a mirror with a 2 meter diameter.

Astronomers have worked very hard to build bigger and bigger telescopes.  Why?
- Want to be able to study faint objects and light gathering power is proportional to square of diameter.  Area = pi r squared
= pi/4 D squared.  Need bigger collecting area to study fainter objects in a reasonable amount of time.
- Also the resolving power improves as the diameter gets bigger.  However on the ground this does not help - once you get a
diameter larger than about 10 cm you are limited by atmospheric "seeing."  Turbulence in Earth's atmosphere limits spatial
resolution from ground to 1 arcsec.  Causes the 'twinkling' of the stars.

Most large optical telescopes are reflectors rather than refractors.  Large refractors would require large lenses of very
high quality - extremely expensive to make.  Also can only support the lens by its edges and it sags under its own weight.

Reflectors are much easier. Only have to grind the front side precisely.  Can support along the back to reduce sagging.
Examples of large reflectors - Palomar Keck HET.

Important optical telescope technology.
Spectrographs - devices that take the light from cosmic objects and spread it out into its component colors.  Make spectra.

People used to use photographic plates to make astronomical images but now use mainly charge coupled devices.  Postage stamp sized electronic detectors with about 1 million pixels.  Kind of like what is used in home movie cameras.  More efficient than photographic plates in light collection and easier since they are already digital.

Segmented mirrors.  Very large mirrors sag under their own weight and are very heavy.  This makes telescopes expensive.  A clever trick is to use many small mirrors to simulate one big one.  See page 76 of book for picture.  HET is segmented 91 hexagonal segments.

Also telescopes at other wavelengths to let us study the Universe in different kinds of light - different colors of light tell us
about different physical conditions.  X-rays - hot.  IR - cool.
Covered on pages 81-87 of book.

A few key concepts.

• Interferometer - 2 or more telescopes observe same object to simulate a telescope as big as the distance between them.  Most successful in radio so far - simulate telescopes as big as the Earth.
• Space astronomy - atmosphere blocks much of electromagnetic spectrum.  IR UV X-rays.  Must launch satellites (e.g.. IRAS IUE Uhuru Einstein ROSAT ASCA AXAF)

Now I will start to go over the cosmology part of this section.

Cosmology - In this section of the course I focused on the 3 pillars of the Big Bang model.

- The idea that the Universe started from a hot dense compact state about 10-20 billion years ago.
- Scientists have worked very hard to test the Big Bang model.

The 3 Pillars are.
- The expansion of the Universe according to the Hubble Law
- The cosmic microwave background
- Primordial nucleosynthesis.

For the test do not worry about flatness problem horizon problem inflationary universe or grand unified theories.  Should
know the rest of Chapter 15 pretty well though.

Expansion of the Universe.
Hubble expansion of the Universe.
v=H₀D
v in km/s
D in Mpc
H₀ = 75 km/s / MPC

Galaxies that are further away are receding from us faster.  We understand that Hubble's Law is a consequence of the expansion of the Universe.  Space itself is expanding carrying the galaxies with it.  If more space between 2 galaxies more space to expand so it moves away faster.

No central point of expansion and no "edge" to Universe - balloon analogy in 2D for our 3D Universe.

Evidence for Big Bang - if expanding now must have been smaller in the past.
We also used the Hubble expansion to estimate the age of the Universe.
2 galaxies separated by D
time when touching = D/v = 1/H₀

We then started talking about the very early universe - from about 300 000 years after the Big Bang and earlier.

During this time Universe was hot and smooth a soup of plasma - free electrons and protons.  Stars planets and galaxies had not yet formed.

We can look back to "see" the radiation emitted by this plasma when the Universe was just 300 000 years old.  Very important
probe of early Universe.  Sky is "glowing" in microwaves with this radiation - cosmic microwave background.
Blackbody spectrum T = 3K.  The plasma that emitted it was much hotter than 3K but radiation gets redshifted - cools radiation to longer wavelength.

Cannot make "pictures" of the Universe any earlier than this.  Can't see through the plasma "fog" that filled the Universe at earlier times.

The COBE satellite made 2 breakthrough discoveries about CBR
- showed precisely a blackbody spectrum - confirmed Big Bang
- made a map of microwave background that revealed intrinsic anisotropy.  Important since it tells us that the Universe was
not perfectly smooth 300 000 years after the Big Bang.  Had density variations at about 1 part in 50000.  These grew by
gravity into galaxies galaxy clusters and even larger scale structures.

The Final Pillar of the Big Bang is
Primordial Nucleosynthesis - tells us about the conditions at even earlier times from 2-17 minutes after the Big Bang.
There is too much He in the Universe to have been made just in Stars.  About 24% of the atomic mass of the Universe in He.

Thought to have been made fusion reactions in the very early
Universe.  Temp was about 300-900 million K and universe was as hot as the core of a star so could burn H->He.

Scientists can use a computer to simulate the nuclear reactions in the early Universe and predict about 24% of helium - good
evidence for Big Bang!

The fate of the Universe.
The density of our Universe will determine its ultimate fate.
If our Universe has an average density of greater than 1x10^-29 g/cc
it will ultimately recollapse = closed universe (death by fire)
if less than 1x10^-29 g/cc it will expand forever = open universe(death by ice)

We think that the Universe is open but doesn't know for certain.  Also the average density determines large scale geometry of
Universe.
> 1xx10^-29 g/cc 3D version of the 2D surface of a sphere
< 1x10^-29 g/cc 3D version of the 2D surface of a sphere.